To study epidermal differentiation and carcinogenesis, cDNA and genomic clones corresponding to the major proteins expressed in mouse epidermis have been isolated and characterized. These include the basal layer keratins K5 and K14, the suprabasal layer keratins Kl and KIO, the hyperproliferative keratins K6 and K16, and K13, expressed in malignant tumors but not in benign tumors or normal epidermis. In addition, cDNA and genomic clones for filaggrin, the protein which organizes keratin filaments into larger bundles, and loricrin, the major precursor protein of the cornified envelope, have been isolated. Monospecific antibodies to the C terminal amino acid sequences, as well as nucleic acid probes to unique regions of the cDNAs, allow the study of the expression of these markers in normal epidermis, benign and malignant tumors, and cells in culture. Recombinant constructs of KI and KIO coding sequences with heterologous promoters were transfected and expressed in mesenchymal cells and neoplastic keratinocytes. KI and K10 filaments did not form in fibroblasts unless cytokeratins K5 and K14 were also expressed. Kl and KIO formed filaments in malignant keratinocytes which express K5 and K14 constitutively. Cells expressing Kl protein, but not KIO protein, continued to proliferate. An epidermal-specific regulatory region in a 12 kb fragment of the human KI gene was localized in recombinant constructs with a reporter gene. A calcium-sensitive positive regulatory region was located 3' to the coding sequence. Deletion analysis identified several shorter elements in the terminal-1700 nucleotides of the 12 kb genomic clone. This regulatory region did not contain sequences responsive to retinoids. Samples of human skin from patients treated with retinoids showed marked changes in the expression of differentiation-related proteins within four days of exposure. Therefore, retinoid treatment of human skin is likely to be associated with functional changes in the epidermis.